Thousands of overseas passengers were stranded at Thailand's main international airport today after all flights were halted because of an invasion by anti-government protesters.

The takeover of Bangkok's Suvarnabhumi airport led to the cancellation of hundreds of flights and represented a dramatic escalation of the demonstrators' efforts to oust the government.

Protesters from the People's Alliance for Democracy (PAD) have been campaigning for six months to remove the government and would like to provoke the army to step in and depose the prime minister, Somchai Wongsawat.

The army chief, Anupong Poachinda, has repeatedly said a military takeover will not resolve the fundamental tensions in Thai society. Today, he called a top-level meeting of senior officers, academics and officials.

The protest turned violent again early today when a grenade was thrown at PAD protesters manning a checkpoint on the road to the airport. Two more grenades were tossed at PAD sentries on roads to Bangkok's second airport, injuring three people.

At one point about 50 masked protesters demanded access to Suvarnabhumi's control tower to check the flight schedule in the belief the prime minister, due to return from an Apec summit in Peru, might consider landing there. No air traffic control staff were in the building to give them the information.

The PAD vanguard - brandishing iron bars and wearing goggles and hard hats - last night pushed through the lines of riot police who were ordered not to use violence after two protesters were killed by faulty teargas canisters in clashes last month.

They stormed into the airport and swirled around the sprawling concourse, first forcing the cancellation of outbound flights and then incoming services. Thai Airways rerouted 16 inbound flights to the old airport at Don Muang, while many other airlines cancelled services.

The airport, an important regional hub, handles around 700 flights each day and almost 40 million passengers a year. Serirat Prasutanont, the airport director, said the action left the country's reputation in tatters. "This incident has damaged Thailand's reputation and its economy beyond repair," he said.

Some of the stranded passengers were being put on coaches today, destined for hotels in Bangkok or the resort town of Pattaya, with little prospect of the airport reopening quickly.

Thousands of demonstrators, mostly sporting yellow shirts or armbands in allegiance to the revered king, settled in on the departure area or milled about the check-in desks as barricades blockaded the roads to and from the airport.

Around 3,000 stranded and bemused passengers were given food handouts by the demonstrators, who apologised for the inconvenience and attempted to explain the motivation for the disruption.

Many were angry and some scared, fearing a confrontation, particularly after a squad of riot police came to the outside of the terminal building at about 5am.

"People have told us to be careful that it could be dangerous," said Patricia Peel, 56, from Portsmouth, whose partner, John Vineal, uses a wheelchair. "But this is absolutely disgusting. I've spoken to many, many English people who said they'd never, ever come to Thailand again."

Passengers were angered by airlines' inability to tell them what was happening or when flights might depart. Some feared going to a hotel in case they missed their plane.

"We came here at 5am," said Jason Melbourne, 40, from Belper, Derbyshire. "But Qatar Airways have told us nothing. I'd happily go and spend a couple of days in Bangkok, but we don't know what to do. They've been useless."